Bravo and Sierra are two of the military's words used to prevent misunderstandings in radio transmission. They stand for the letters B and S in the same way that Alpha means A and X-Ray means X.

Using "I call bravo sierra" is a more articulate and less crude way of saying "I call bullshit". It also can be a coded way of letting your friends know what you think while keeping the clueless out of the circle.

The phrase has been popularized by the XM Satellite radio team of Opie & Anthony.